Thaw the spinach, then squeeze out as much water as possible.
Combine spinach, sour cream, mayonnaise and horseradish in a mixing bowl; you can also do this in a food processor if you like a more pureed consistency (I often do it by hand, or do it VERY CAREFULLY in the food processor so as not to over-process).
Stir in the dry soup mix, the chopped water chestnuts and the onion; stir well.
Refrigerate for at least 2 hours to let flavours meld.
Before serving, stir shrimp into dip mixture.
Cut off the top of the loaf of bread and hollow it out, reserving the lid and the inner bread chunks to serve alongside.
Fill hollowed bread shell with dip and serve with the bread, cut into chunks, along with an assortment of crackers and carrot sticks.